The invention relates to a light image reading method for reproducing and transmitting with higher fidelity the image signal read from an original document by the use of an array of photoelectric converting elements such as photodiodes.
In light image reading methods, such as original document reading methods used in facsimile equipment, an original document is scanned so that its light image is converted into a series of electric signals which in turn are compared with a predetermined level of voltage to be converted into binary signals. For example, a photodiode array having a number of photodiode elements arranged in the main scanning direction is used to scan the original document so as to provide a series of binary signals in accordance with the image information on the original document. In order to produce an accurate copy from the original document, it is conventional practice to provide double the number of photodiode elements in the scanning array as there are image units on a scanning line so that the photodiode arrangement period is one-half the minimum image period. However, this results in a very expensive method as the number of photoelectric converting elements is increased.
Therefore, the present invention has for its object to provide a light image reading method or an image signal converting method using a photodiode array comprising photodiode elements equal in number to the maximum number of image units on one scanning line of the original document so that the element arrangement period is equal to the maximum image frequency, and logically operating on the output image signal from the photodiode array to double the number of image units. The method of the present invention can relatively inexpensively provide improved image signal resolving power and halftone reproduction.